Mycotoxins are secondary metabolites produced by filamentary fungi. An important representative of mycotoxins is zearalenone (ZEN), which was previously known as F-2 toxin, which is produced by a variety of Fusarium fungi and can be found throughout the world. These fungi infest cultivated plants, among others, such as various types of grain, wherein the fungal infestation usually occurs before the harvest when the growth of the fungi and/or the mycotoxin production may take place before storage or may even take place after harvest, either prior to storage or under improper storage conditions. The FAO has estimated that 25% of agrarian products throughout the world are contaminated with mycotoxins, thus resulting in substantial economic losses. In an international study concluded recently, a total of 23,781 samples were analyzed from January 2009 to December 2011, 81% of them testing positive for at least one mycotoxin and 45% testing positive for ZEN. ZEN has been found in all regions of the world and in all types of grain and feed crops tested, such as corn, soy flour, wheat, wheat bran, DDGS (dried distillers grains with solubles) as well as in finished animal feed mixtures with an incidence of up to 100%.
ZEN is a nonsteroidal estrogenic macrocyclic lactone with the following structural formula, synthesized by way of the polyketide metabolic pathway:
and its name according to the IUPAC nomenclature is (2E,11S)-15,17-dihydroxy-11-methyl-12-oxabicyclo[12.4.0]octadeca-1(18),2,14,16-tetraene-7,13-dione.
However, a variety of ZEN derivatives also occurs in nature and may be formed by enzymatic or chemical modifications of ZEN. Examples include glycosidic ZEN conjugates or those containing sulfate, formed by fungi, plants or a mammalian metabolism as well as ZEN metabolites formed in the human or animal organism, among others. ZEN derivatives are understood below to be ZEN conjugates or ZEN metabolites that occur naturally or are synthesized by chemical or biochemical synthesis but in particular α-zearalenol (α-ZEL; (2E,7R,11S)-7,15,17-trihydroxy-11-methyl-12-oxabicyclo[12.4.0]-octadeca-1(18),2,14,16-tetraen-13-one), β-zearalenol (β-ZEL; (2E,7S,11S)-7,15,17-trihydroxy-11-methyl-12-oxabicyclo[12.4.0]octadeca-1 (18),2,14,16-tetraen-13-one), α-zearalanol (α-ZAL; (7R,11S)-7,15,17-trihydroxy-11-methyl-12-oxabicyclo[12.4.0]octadeca-1(18),14,16-trien-13-one), β-zearalanol (β-ZAL; (7S,11S)-7,15,17-trihydroxy-11-methyl-12-oxabicyclo[12.4.0]octadeca-1 (14),15,17-trien-13-one), zearalenone 14-sulfate (Z14S; [(2E,11S)-15-hydroxy-11-methyl-7,13-dioxo-12-oxabicyclo[12.4.0]octadeca-1(18),2,14,16-tetraen-17-yl]hydrogen sulfate), zearalenone-14-glycoside (Z14G; (2E,11S)-15-hydroxy-11-methyl-17-[(3R,4S,5S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)-tetrahydropyran-2-yl]oxy-12-oxabicyclo[12.4.0]octadeca-1(18),2,14,16-tetraene-7,13-dione) as well as zearalanone (ZAN; (11S)-15,17-dihydroxy-11-methyl-12-oxabicyclo-[12.4.0]octadeca-1 (18),14,16-triene-7,13-dione).
ZEN as well as ZEN derivatives, in particular α-ZEL, β-ZEL, Z14S, α-ZAL, β-ZAL, Z14G and ZAN can also be detected in processed foods and animal feed products, such as bread or beer because of their high chemical and physical stability.
ZEN binds to the estrogen receptor and can cause hormonal disruptions, being absorbed immediately after oral ingestion and converted by mammals into the two stereoisomeric metabolites α-ZEL and/or β-ZEL. For example, α-ZEL but also α-ZAL and/or ZAN have a much stronger estrogenic effect than ZEN. Meanwhile, conjugated ZEN derivatives have a lower estrogenic activity than ZEN but ZEN can be released again from these ZEN derivatives in the digestive tract under some circumstances.
Although ZEN has a relatively low acute toxicity and has an oral LD50 of up to 20,000 mg/kg body weight, subacute and/or subchronic toxic effects such as teratogenic, carcinogenic, estrogenic and immunosuppressant effects may occur in animals or humans with prolonged exposure. Feed contaminated with ZEN leads to developmental disorders in mammalian animals, but pigs, in particular piglets, are extremely sensitive to ZEN. ZEN concentrations of more than 0.5 ppm in feed result in developmental disorders, and concentrations of more than 1.5 ppm in pigs, for example, can result in hyperestrogenicity, and concentrations of 12 ppm ZEN have been blamed for spontaneous abortions in cattle. Since zearalenone is absorbed rapidly through the mucous membranes, in particular through the gastric mucosa as well as the oral mucosa, immediate and quantitative deactivation is essential. ZEN can be detected in blood even 30 minutes after oral administration. In this case, the use of isolated enzymes offers some advantages with respect to microorganisms, such as a higher specific activity or a quicker effect. Because of the harmful effects of ZEN, the European Union has binding upper limits for ZEN in foodstuffs as well as recommendations for upper limits for ZEN in animal feed products (EC No. 1881/2006).
The primary strategy for reducing ZEN contamination of foods and animal feed products is to restrict the growth of fungi, for example, by maintaining “good agricultural practice.” This includes, among other things, ensuring that the seed is free of pests and fungal infestation or that agricultural waste products are removed from the field promptly. In addition, fungal growth in the field can be reduced through the use of fungicides. After the harvest, the harvested material should be stored at a residual moisture level of less than 15% and at a low temperature to prevent the growth of fungi. Likewise, material contaminated by fungal infestation should be removed before further processing. Despite the list of measures, I. Rodriges and K. Naehrer (2012) have reported that, even in regions with the highest agricultural standards, such as the United States and Central Europe in the years 2009 to 2011, 29% and 39% respectively, of the tested corn samples were contaminated with ZEN.
Additional possibilities for removing ZEN from foodstuffs or animal feed products include adsorption and/or transformation of the mycotoxin. This requires that binding of the mycotoxin to the adsorbent must be strong and specific over a wide pH range and must remain stable in the gastrointestinal tract. Although some nonbiological adsorbents such as activated carbon and silicates or synthetic polymers such as cholestyramine can be used efficiently for aflatoxins, their use for other mycotoxins is limited. The main disadvantage of adsorbents is the nonspecific binding of other molecules, which are in some cases essential for nutrition. Biological adsorbents such as yeast or yeast extracts have also been described in the literature but have a limitation similar to that of nonbiological adsorbents.
Detoxification of ZEN by physical and chemical treatments is also limited. ZEN cannot be deactivated effectively by thermal treatment, but the ZEN content can be reduced by 83.9% by extrusion and treatment with oxidizing agents, for example, for 16 hours at 80° C. with 10% hydrogen peroxide solution. Use of extrusion methods and oxidizing agents such as ozone or hydrogen peroxide in the production of foodstuffs and animal feed products is limited because of the high cost, the loss of quality and in some cases the low efficacy and low specificity.
Biotransformation of ZEN by means of microorganisms such as Trichosporon mycotoxinivorans, Gliocladium roseum or Bacillus subtilis strains and/or enzymes isolated from them such as hydrolases or peroxidases his described, for example, by E. Vekiru et al. in Appl. and Environ. Microb., 2010, 76, 7, 2353-2359.
EP 0 938 575 B1 has described ZEN-degrading properties of bacteria of the genus Rhodococcus or Nocardia, in particular R. globerulus, R. erythropolis and N. globerula. 
WO 02/076205 describes the ZEN-degrading effect of enzymes isolated from Gliocladium roseum, including α,β-hydrolase and zearalenone hydrolase 1 (ZHD1), which catalyze the degradation of ZEN by means of a catalytic triad.
WO 2012/113827 discloses recombinant zonases, namely enzymes that degrade ZEN and remain stable in the gastrointestinal tract. These include microorganisms such as Thermobifidia fusca, Streptomyces exfoliates, Acidovorans delafieldii and Streptomyces sp. in particular.
Polypeptides or enzymes capable of hydrolyzing ZEN and/or at least one ZEN derivative may also be designated as zonases.
The terms used hereinafter are taken from the technical language and each is used in the traditional meanings, unless something to the contrary is indicated. Thus, for example, the term “polynucleotide” relates to all types of genetic material of all lengths and sequences such as single-stranded and double-stranded DNA and RNA molecules, including regulatory elements, structural elements, groups of genes, plasmids, entire genomes and fragments thereof. The designation “polypeptide” includes proteins such as, for example, enzymes, antibodies as well as polypeptides with up to 500 amino acids, such as, for example, peptide inhibitors, domains of proteins or also short polypeptides with short sequence lengths, for example, less than 10 amino acids, such as receptors, ligands, peptide hormones, tags and the like. The designation “position” in a polynucleotide or polypeptide relates to a single specific base or amino acid in the sequence of the polynucleotide or of the polypeptide.